University of Lübeck, Medical Laser Center Lübeck GmbH Optical Coherence Tomography

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1 University of Lübeck, Medical Laser Center Lübeck GmbH Optical Coherence Tomography 3. The Art of OCT Dr. Gereon Hüttmann / 2009

2 System perspective (links clickable) Light sources Superluminescent diodes Semiconductor amplifiers Femtosecond lasers Beam delivery and probes Hand-held probe Catheter Ophthalmoscope Microscope OCT imaging engine Resolution Reference delay scanning Doppler/polarization/spectroscopy Detection Frequency domain Computer control Drive system Real-time display Data management Image & signal processing Motion reduction Speckle reduction Image enhancement Rendering algorithms

3 Optical Coherence Tomography Base Technology Applications devices Clinical Applications

4 The Four Families of OCT Sampling Parameter Spectrum SNR n S Coherence function SNR n l / z S c max Time multiplexed (point detector) Swept Source (OFDR) Demanding light source Phase Noise FFT necessary TD-OCT Demanding phase mod. Phase Noise Reduced SNR Space multiplexed (line detector) SD-OCT Good optics needed Limitations due to detector performance FFT necessary Linear OCT Limitations due to detector performance Limited z-range Reduced SNR

5 Time Domain OCT Faserkoppler Photodiode1 - Photodiode Bandpaßfilter Faserkoppler Phasenmodulator Photodiode2 Lichtquelle Gleichrichter Bandpaßfilter Gleichrichter Tiefpaßfilter Computer Probenarmoptik Tiefpaßfilter Probe Fiber optical interferometer -> stable, flexible Sophisticated signal processing -> measurement over 10 orders of magnitude

6 The Reference Delay Line Linear translated mirror Multipass translated mirror

7 Rotation based delay-line

9 Fiber Streching

10 Light sources for OCT Continuous sources SLD/LED/superfluorescent fibers, center wavelength; 800 nm (SLD), 1300 nm (SLD, LED), 1550 nm, (LED, fiber), power: 1 to 10 mw (c.w.) is sufficient, coherence length; 10 to 15 mm (typically), Pulsed lasers mode-locked Ti:Al2O3 (800 nm), 3 micron axial resolution (or less).

11 Superluminescent diodes (SLDs) Definition: broadband semiconductor light sources based on superluminescence (Acronym: SLD) Superluminescent diodes (also sometimes called superluminescence diodes or superluminescent LEDs) are optoelectronic semiconductor devices which are emitting broadband optical radiation based on superluminescence. They are similar to laser diodes, containing an electrically driven p-n junction and an optical waveguide, but lack optical feedback, so that no laser action can occur. Optical feedback, which could lead to the formation of cavity modes and thus to pronounced structures in the spectrum and/or to spectral narrowing, is suppressed by means of tilting the output facet relative to the waveguide, and can be suppressed further with anti-reflection coatings. Superluminescence: amplified spontaneous emission

12 TD-OCT Faserkoppler Photodiode1 - Phasenmodulator Photodiode2 Faserkoppler Lichtquelle Bandpaßfilter Gleichrichter Tiefpaßfilter Computer Probenarmoptik Probe

13 FD-OCT (Spectral Radar) Refenenz SLD Objekt FFT Spektrometer

14 Signals in FD-OCT I( k) I I 2 I I cos k z S R S R

16 Phase-resolved FDOCT Sarunic MV, Choma MA, Yang CH, et al. Instantaneous complex conjugate resolved spectral domain and swept-source OCT using 3x3 fiber coup OPTICS EXPRESS 13 (3): FEB

17 Removing the Depth Ambiguities

18 Das Spektrometer Linear image Sensor mit 1024 (1) Pixel, 8µx125µm (2) 180nm Spektralbereich Auflösung 0.1 nm Messtiefe 1 mm FWHM 4 µm 150 (3) A-Scans (4) pro Sekunde SNR = 108 db (7) (9)

19 Image Sensors Adressierungslogik Adressierungslogik A:Passiv Pixel CMOS Sensor Adressierungslogik B: Active Pixel CMOS Sensor C: CCD Sensor

20 Signal [electrons] Relative amplitude Spatial Resolution of the Detector CMOS-Sensor LIS pixel 518 pixel 519 pixel 520 1, , , , ,2 0 a) Lateral position of focus [µm] 0, Spatial frequency [cycles/pixel]

21 Roll-Off of the Sensitivity

22 Noise [elektrons] Noise performance of the Detector NMOS Hamamatsu 3904 Hamamatsu 3903 CMOS (LIS 1024) quantum noise limit E7 1E8 Average charge per pixel [elektrons]

23 Signalaufbereitung

24 Spectral Shaping of the Spectrum (White Adjust) Hanning window

Hanning window Tapered Cosine Multi gauss Noisy 120 100

1,5 60 1,0 40 0,5 0,0 0 200 400 600 800 1000 Wavelength [a.

25 Intensity [a.u] Amplitu Spectral Shaping of the Spectrum 4,0 3,5 3,0 Hanning window Tapered Cosine Multi gauss Noisy Hanning window Tapered Cosine Multi gauss Noisy 2,5 80 2,0 1,5 60 1,0 40 0,5 0, Wavelength [a.u] 20 0,86 0,88 0,90 0,92 0,94 0,96 0,98 1,00 Pathlengthdifference [mm] Hanning window Tapered cosine window Multi gauss

26 Transformation to k=2 /

27 Swept-Source OCT

28 Swept Soures for OCT Scanning sources tune narrow-width wavelength over entire spectrum, resolution similar to other sources, advantage that reference arm is not scanned, advantage that fast scanning is feasible.

29 Multimode Operation

31 Fourier Domain Mode Lock (FDLM) Dispersice Pulse Streching

33 Dispersion in the Interferometer

34 4 2 optical path lengths and chromatic dispersion 1 L c Probe Photodiode No additional optics

35 4 3 Optical Path Lengths and Chromatic Dispersion 1 n g d Probe d Photodiode Additional optics with n g = n - l dn/dl 1

36 4 4 Optical Path Lengths and Chromatic Dispersion Probe d Photodiode Correction in the reference arm

37 4 5 Optical Path Lengths and Chromatic Dispersion 1 2 c L ( c l D d ) 0 chr Probe d Photodiode Group velocity dispersion D chr = 1/c 0 dn g /dl 0

38 46 Optical Path Lengths and Chromatic Dispersion Probe d Photodiode Correction by introducing GVD in the reference arm

39 47 Conditions for Balancing the Optical Path and GVD Balancing of optical path length probe n g d i n g reference d i Balancing of group velocity dispersion probe D d D d chr i chr i reference

40 48 Group Index of Refraction and Group Velocity Dispersion at 1300 nm D Material n chr g [ps/nm/km] Air 1,00 0 Fused Silica 1,46 2,6 BK7 1,52-2,32 Water (20 C) 1,34-19,4 (37 C) 1,34 23,9 BaSF1 1,62-42,3 SF57 1,83-115,9 GRIN-Lens EG27 (NSG) 1,53-47,7 ZD-fiber n. m. 0 NZD-fiber (LEAF) n. m. -24,3

41 49 n p, n g D chr [psec/nm/ km] Chromatic Dispersion of Fused Silica 1,48 0 1,47 n g -200 Quarz 1, SF57 1,45 n p ,44 0,8 0,9 1 1,1 1,2 1,3 1,4 Wellenlänge [µm] ,8 0,9 1 1,1 1,2 1,3 1,4 Wellenlänge [µm]

42 index of refrection 50 OCT Endoscope graded index lense (GRIN) radius GRIN lense in mm CCD-camera scanner 3 mm 0 endoscope 270 mm length

43 51 Compensation of GVD Length of NZD-fiber [mm] NZD fibre length inside the reference = 1,15 m

44 52 Compensation of GVD Length of NZD-fiber [mm] NZD fibre length inside the reference = 1,15 m

45 Application Systems Galvanometerscanner Collimator Lens Beamsplitterplate Probe

47 Micromotor Endoscope Herz PR, Chen Y, Aguirre AD, et al. Micromotor endoscope catheter for in vivo, ultrahigh-resolution optical coherence tomography OPTICS LETTERS 29 (19): OCT

Optical coherence tomography

Optical coherence tomography Peter E. Andersen Optics and Plasma Research Department Risø National Laboratory E-mail peter.andersen@risoe.dk Outline Part I: Introduction to optical coherence tomography